In recent years, application of infinite solar light producing no harmful substances has been actively studied. Inorganic type solar cells such as single crystalline silicon, polycrystalline silicon, amorphous silicon, and cadmium telluride and indium copper selenide for domestic use are provided as those presently available in practical use as a clean energy source in application of solar light.
However, drawbacks of these inorganic type solar cells include in the case of the silicon type, not only extremely high purity is required, but also complicated purification process includes many steps at high production cost.
On the other hand, many solar cells employing an organic material have also been proposed. Examples of the organic solar cell include a Schottky type photoelectric conversion element in which a p-type organic semiconductor and metal having a small work function are joined, and a heterojunction type photoelectric conversion element in which a p-type organic semiconductor and an n-type inorganic semiconductor or a p-type organic semiconductor and an electron acceptable organic compound are joined. The utilized organic semiconductors are synthesized dyes or pigments such as chlorophyll, perylene and so forth, conductive polymers such as polyacetylene and so forth, and the composite material thereof. The material to be used as the cell material is thin-layered by a vacuum evaporation method, a casting method, a dipping method or the like. The organic materials have advantages of low cost and easy production of large area, but there is a problem such as a low conversion efficiency of 1% or less together with insufficient durability.
In such the situation, a solar cell exhibiting favorable properties has been reported by Dr. Gratzel et al. in Switzerland, cf. Non-patent document 1 for example. The proposed cell is a dye sensitizing type solar cell, and a wet type solar cell in which a porous titanium oxide thin film spectrally sensitized by a ruthenium complex is provided as a functional electrode. Advantages of this technique are that purification of an inexpensive oxide semiconductor such as titanium oxide up to high purity is not necessary, and solar light having a large visible light component can be effectively converted into electricity in accordance with usable light covering a wide wavelength of visible light at low cost.
In contrast, ruthenium complex is under threat of its supply in cases where this solar cell is put into practical use, since the ruthenium complex as limited resource is utilized. Further, since ruthenium complex is expensive, and produces a problem in aging stability, this problem can be solved if the ruthenium complex can be replaced by an inexpensive and stable organic dye.
It is disclosed that a compound having a rhodanine moiety-containing amine structure is utilized as the dye to obtain an element exhibiting high photoelectric conversion (refer to Patent Document 1, for example). However, since photoelectric conversion efficiency in this case is lower than in the case of use of a ruthenium complex, even though employing this dye, a sensitizing dye exhibiting high photoelectric conversion efficiency is further desired to be obtained.